System manufacturers that integrate processors into their systems have various goals with respect to power consumption and thermal considerations. For example, the system manufacturer may want to advertise that their system can provide a minimum battery lifetime. The power consumption of the processor is a major determinant of the battery lifetime. For another example, the system manufacturer implements a particular thermal solution (e.g., heat sinks, fans, venting). Again, the power consumption of the processor is a major determinant of whether the thermal solution can keep the temperatures of various system components (e.g., keyboard, processor, chipset) below required tolerances.
In order to meet these goals, the system designer specifies to the processor designers a maximum power value that the processor is allowed to consume, referred to herein as P. For example, the value of P might be 8 Watts in a relatively low-power system, such as a hand-held device, PDA, or sub-notebook computer. In order to comply with the system manufacturer's requirement, the processor designer designs the processor to never operate above a performance point—generally speaking, a frequency/voltage combination—that would cause it to exceed the P value even if the workload placed on the processor by the currently running software application mix is at its highest.
However, depending upon the system configuration, such as the power settings chosen by the user, the operating system may command the processor to transition between running states and sleep states at different times, based on the workload of the currently executing software applications. For example, the operating system may command the processor to be in a sleep state 90% of the time and to operate in a running state only 10% of the time. In such situations, the processor consumes significantly less than the P value specified by the system manufacturer. For example, the processor may consume at a rate of 1.2 Watts while in a sleeping state rather than the maximum 8 Watts specified by the system manufacturer, and even in the running state the processor may consume power at a rate of less than the maximum 8 Watts, depending upon the workload imposed by the software.